1. Field of the Invention
This invention relates to an apparatus for processing information optically, and more particularly to an optical information-processing apparatus which is provided with a multi-beam optical head for simultaneously recording, retrieving and erasing information by the use of multi-light beams.
2. Description of the Related Art
There have been recently developed optical information processing apparatuses such as optical disc apparatuses which record image information of documents or the like, retrieve the required image information and reproduce the same as hard copies or soft copies. With the optical disc apparatus, a light beam is converged on a disc-shaped recording medium, for example, an optical disc, and information is recorded on the recording medium or retrieved therefrom. In more detail, in a recording mode, a light beam is modulated and the modulated light beam is converged on the optical disk so that the condition of the recording medium is changed, for example, so that pits are formed on the recording medium to record information on the optical disk. In a retrieving mode, a light beam having a constant light intensity is converged on an information recording medium and the intensity is modulated by the changed conditions of the recording medium or the pits according to the recorded information. The light beam thus modulated is detected and converted into electrical signals. By processing the signals, the information is reproduced. During recording and retrieving, the optical disc is rotated at a constant linear speed, and the light beam converged on the optical disc is linearly moved in the radial direction of the optical disc.
As one of the optical disc apparatuses, an optical information-processing apparatus has been recently developed which is provided with a multi-beam optical head for recording and retrieving information simultaneously by employing a plurality of light beams. FIG. 1 shows an example of the multi-beam optical head of such a conventional optical disc apparatus.
Referring to FIG. 1, recording light beams L1 and retrieving light beams L2 are emitted from a semiconductor laser array 1, transmitted through a beam splitter 2 and are converged on the recording surface 5A of an optical disc 5 through a reflecting prism 3 by an objective lens 4. In this case, it is preferred that the light beams L1 and L2 should be arranged such that their converged points lie adjacently to each other on the same track. With this arrangement, the recording and retrieving beam spots which are adjacent to each other are formed on a predetermined track on the recording surface. The optical disc on which the recording and retrieving beam spots are thus formed is rotated at a constant linear speed, whereby the predetermined recording area is first illuminated by the recording light beams and immediately thereafter by the retrieving light beams. In other words, retrieving is carried out immediately after recording. The light beams L1 and L2 converged on the recording surface 5A are reflected thereby and are returned to the beam splitter 2 through the objective lens 4 and the reflecting prism 3. Thereafter, both light beams L1 and L2 are reflected on the boundary face in the beam splitter 6 to be incident on a beam splitter 6. Part of each of the light beams L1 and L2 passes through the beam splitter 6 and is incident on a photo-detector 7A which generates a tracking signal for controlling the light beams L1 and L2 and maintaining the light beams to trace the predetermined area of the optical disk, that is, the predetermined track. The remaining parts of the light beams L1 and L2 are reflected from the boundary face in the beam splitter 6 and pass through a convex lens 12 disposed on the light beam emerging surface of beam splitter 6. The light beams converged by the convex lens 12 are incident on a spatial filter 8 and are spatially separated into recording light beams and retrieving light beams. The recording beams are reflected from the spatial filter 8 and are converged on a photo-detector 7C. The retrieving beams pass through a pin hole 8A and are detected as a retrieving signal and a focusing control signal by photo-detectors 7B, 7D.
Alternatively, there has been known an optical information-processing apparatus which is provided with a multi-beam head for emitting a retrieving beam and a recording beam having different wavelengths from each other. With this apparatus, the recording light beams and retrieving light beams having different wavelengths from each other and emitted from respective light sources are synthesized by a dichroic mirror and are converged by an objective lens on the predetermined area of an optical disc. It is preferred that both beams L1 and L2 should be converged on the same track in such a manner that their converging points are arranged adjacent to each other. It mean that the adjacent small recording and retrieving beam spots are formed on the predetermined track on the recording surface. Similarly to the apparatus of FIG. 1, the converged beam spots are utilized to record information on the optical disc and to retrieve information therefrom. The light beams are reflected on the optical disc and are incident on a dichroic mirror to be divided into a recording light beam and a retrieving light beam. The divided beams generate an information signal and a focus control signal.
With the apparatus in which the retrieving light beam and the recording light beam are divided by the spatial filter having the pin hole, the light beams converged on the photo-detector through the pin hole are used to retrieve the information and to control the focus. In this arrangement, light beams must pass through the pin hole, and thus the spatial filter provided with the pin hole must be set at a high accuracy. The necessity of providing an optical system for focusing the light beam passing through the pin hole hinders the whole size of the head from being minimized. Further, since cross talks occur between the recording light beam and the retrieving light beam, the retrieving signal and the focus control signal cannot be accurately detected.
With the multi-beam optical head using two kinds of light beams consisting of the retrieving and recording light beams having different wavelengths from each other, on the other hand, it is difficult to form laser diode structures as the light sources in a single chip, and the two laser diode chips must be disposed closely adjacent to each other at a high mounting accuracy. Further, the usage of light beams having different wavelengths from each other requires the correction of chromatic aberrations of the lens occurring from the difference of the resolving powers of the optical systems.
With the apparatus for processing information optically, the retrieving light beam is converged by the optical detector for generating the focus control signal. The focus control signal is supplied to an objective lens drives circuit which drive a voice coil for driving the objective lens so that the focusing errors of the objective lens are corrected. As shown in FIGS. 2A to 2C, the focus control signal is generated by detecting the difference between the light intensities of beam spots on two photo-detecting areas 20A and 20B. In more detail, the generated focus control signal corresponds to the difference between the light intensity of the beam spot converged on the photo-detecting area 20A of the photo-detector 20 and the light intensity of the beam spot converged on the photo-detecting area 20B of the optical detector 20.
With the conventional focus control apparatus having a multi-beam optical head, two light sources are energized to emit two light beams but one of light beams forms a beam spot on the photo-detecting areas 20A and 20B and the intensity difference between the beam spot portions on the photo-detecting areas 20a and 20B is converted into focus control signal. When the objective lens is greatly deviated from the focusing state at the start of the focus control, that is, when the information recording medium is not in the range of the focal depth of the objective lens, two large images Sr and Sw are formed by the recording and retrieving light beams, as shown in FIGS. 2A and 2C. This creates a problem that the focus detecting sensitivity is lowered. In other words, when the objective lens is greatly separated from the information recording medium, thereby causing a large focus error, only a semicircular beam spot Sr produced by the retrieving light beams Lr is formed on the photo-detecting area 20A (FIG. 2A). To the contrary, when the objective lens is disposed too closely to the information recording medium, thereby causing a large focus error, a semicircular beam spot Sr is formed on the photo-detecting area 20B on the optical detector 20, and at the same time, a semicircular beam spot Sw produced by the recording light beams Lw is also formed on the optically detecting area 20A (FIG. 2C). It follows that the focus control signal which is generated when the objective lens is disposed far from the information recording medium are asymmetrical with that which is generated when the objective lens is disposed too closely to the information recording medium with respect to the zero level signal which is generated when the objective lens is maintained in the just focusing state (FIG. 3). This creates a further problem that the focusing of the objective lens cannot be accurately controlled